SYSTEM DESCRIPTION
The function of a radio or "wireless" system is to send
information in the form of a radio signal. In this presentation,
the information is assumed to be an audio signal, but of
course video, data, or control signals can all be sent via
radio waves. In each case, the information must be
converted to a radio signal, transmitted, received, and
converted back to its original form. The initial conversion
consists of using the original information to create a radio
signal by "modulating" a basic radio wave. In the final
conversion, a complementary technique is used to
"demodulate" the radio signal to recover the original
information.
A wireless microphone system consists generally of
three main components: an input source, a transmitter,
and a receiver.
(See Figure 2-1.) The
input source provides
an audio signal to the
transmitter. The
transmitter converts the
audio signal to a radio
signal and "broadcasts"
or transmits it to the
surrounding area.
The receiver "picks
up" or receives the radio signal and converts it back into an
audiosignal. Additional system components include
antennas and, possibly, antenna cables and distribution
systems. The processes and the basic components are
functionally similar to commercial radio and television and
other forms of radio communications. What differs is the
component scale and the physical system configurations.
There are four basic configurations of wireless
microphone systems, related to the mobility of the
transmitter and receiver components, as required for
different applications. The first configuration involves a
portable transmitter and a stationary receiver. The
transmitter is usually carried by the user, who is free to
move about, while the receiver is located in a fixed
position. The input source in this setup is normally a
microphone or an electronic musical instrument. The
receiver output is typically sent to a sound system,
recording equipment, or a broadcast system. This is the
configuration of the standard "wireless microphone" and is
the arrangement most widely used in entertainment,
public address, and broadcast applications.
The second configuration employs a stationary
transmitter and a portable receiver. In this case, the user
carries the receiver, while the transmitter is fixed. The input
source to the transmitter for these setups is usually a
sound system, playback system, or other installed source.
The output of the receiver is typically monitored through
headphones or loudspeakers. It may feed a portable
audio or video recorder. This is the configuration of
wireless systems for in-ear-monitors, (IEMs) interruptible
foldback systems (IFB), assistive listening, simultaneous
translation, and various instructional uses. It is also, of
course, the configuration of commercial radio and
television broadcast systems when the receiver is mobile
such as a personal radio or a car radio.
The third configuration consists of both a portable
transmitter and a portable receiver. The users of both
components are free to move about. Again, the input
source is usually a microphone and the output is often a
headphone. This is the configuration of "wireless
intercom" systems, though each user in a typical setup
has both a transmitter and a receiver for two-way
communication.
Another application
of this configuration
is for transmission
of audio from
a wireless
microphone to a
portable camera/
recorder in
broadcast, film, and
videography.
The fourth configuration comprises a transmitter
and a receiver that are each stationary. Such setups
are often referred to as "point-to-point" wireless
systems. The typical input would be a playback source
or mixer while the output might be to a sound system
or to a broadcast facility. Examples of this setup are
wireless audio feeds to multiple amplifier/loudspeaker
arrays for temporary distributed sound systems, radio
remote-to-studio links and of course commercial and
non-commercial broadcasts from fixed transmitters to
fixed receivers.
INPUT SOURCES
The input source is any device that provides a suitable
audio signal to the transmitter. "Suitable audio signal"
means an electrical signal within a certain frequency range
(audio), voltage range (microphone level or line level), and
impedance range (low or high) that can be handled by the
transmitter. Though this places some limits on input
sources, it will be seen that almost any type of audio
signal can be used with one system or another.
The most common input source is a microphone,
which may take any one of a variety of forms: handheld,
lavaliere, headworn, instrument-mounted, etc. The audio
signal provided by this source is audio frequency,
Selection
and Operation
of Wireless Microphone Systems
8
CHAPTER 2
Basic Radio Systems
Figure 2-1: general radio system diagram